When using CMake 3.19 or newer and no CMAKE_CXX_STANDARD has been set before the call to find_package(dd4hep), downstream code will error with:
    ```
    CMake Error at /afs/cern.ch/user/v/vavolkl/cmaketest/tmp/CMakeFiles/CMakeTmp/CMakeLists.txt:13 (add_executable):
      CXX_STANDARD is set to invalid value 'CXX_EXTENSIONS'

    CMake Error at CMakeLists.txt:3 (try_compile):
      Failed to generate test project build system.
    ```
    because ${CMAKE_CXX_STANDARD} is empty. I couldn't find any relevant information in the changelog, so this might be a bug in CMake proper. When configuring dd4hep, CMAKE_CXX_STANDARD is assumed to always be set.

Also, don't add particle as daughter to its parents.

This addresses an issue with fully duplicated entries in the parents and
daughters relations for MCParticles. After this bugfix, only a single
entry for each parents is entered in the parents relations, similar for
daughters. More demonstrated behavior in pull request comments.

BEGINRELEASENOTES
- edm4hep: don't add particle as parent to its daughters, and vice versa
ENDRELEASENOTES

As it stands, this project requires to find a python version matching
the one ROOT was build against EXACTLY. This has been discussed in #842.
This adds an option `DD4HEP_RELAX_PYVER` that will accept **any** python
version. It will then perform a custom check if major and minor version
of the ROOT python version and the one found match (in which it should
be safe to assume ABI compatibility). If not, it prints a warning but
continues with the build.

warnings like
-------- WWWW ------- G4Exception-START -------- WWWW -------
*** G4Exception : em0044
      issued by : G4VEnergyLossProcess::SetMinKinEnergy
Parameter is out of range: 0 it will have no effect!
  Process hIoni  nbins= 84 Emin(keV)= 0.1 Emax(GeV)= 100000
*** This is just a warning message. ***
-------- WWWW -------- G4Exception-END --------- WWWW -------

-------- WWWW ------- G4Exception-START -------- WWWW -------
*** G4Exception : em0044
      issued by : G4VEnergyLossProcess::SetDEDXBinning
Parameter is out of range: -2.14748e+09 it will have no effect!
  Process hIoni  nbins= 84 Emin(keV)= 0.1 Emax(GeV)= 100000
*** This is just a warning message. ***
-------- WWWW -------- G4Exception-END --------- WWWW -------

This doesn't happen automatically in EDM4hep but does in LCIO!

Per #883 diquarks are treated as stable by geant4 but they can have nominally non-zero travel times inside an event graph and should be explicitly rejected.

Fix compatibility issues with Geant4 11. See issue in githib: https://github.com/AIDASoft/DD4hep/issues/881

Fix compatibility issues with Geant4 11. See issue in githib: https://github.com/AIDASoft/DD4hep/issues/881

Fix compatibility issues with Geant4 11. See issue in githib: https://github.com/AIDASoft/DD4hep/issues/881

Throw exception if a DetElement child is not present. (See issue https://github.com/AIDASoft/DD4hep/issues/878)